Plastic encapsulated devices are a typical type of packaged device used in the semiconductor industry. In plastic encapsulated devices, semiconductor chips or devices are attached to laminate substrates or conductive metal lead frames. The chips are attached using for example, thermoset epoxies, solder alloys, or by eutectic wetting. The semiconductor chips are then connected to conductive leads using wire bonds and/or conductive clip structures. The assembly is then encapsulated with an epoxy mold compound and separated into individual packaged units.
Improving the adhesion between the epoxy mold compound and the lead frame or substrate continues to be a major challenge in semiconductor packaging. The long term reliability of a device is put at risk when the epoxy mold compound begins to separate from the lead frame or substrate surface. This separation can induce stress on the semiconductor chip, which can result in the chip separating from the lead frame or substrate, or result in the chip itself cracking. Additionally, when the epoxy mold compound separates, a path for moisture or other contaminates is created, which can further impact the reliability of the packaged device.
Various techniques have been tried to improve the adhesion characteristics of epoxy mold compounds. The techniques include using adhesion promoter additives in the epoxy mold compounds; bead blasting, laser ablating or etching the lead frames or substrates to remove material from their surfaces; removing material to form mold lock structures, or adding dummy structures to the lead frames such as wire bonds. However, these approaches have had limited success, and adhesion problems still exist and continue to be a problem in the plastic encapsulated packages.
Accordingly, a structure and method of assembly are need that further improve the adhesion between epoxy mold compounds and conductive lead frames and/or laminate substrates.